1. Field of the Invention
The present invention relates to a method of measuring the depth of a trench for use in forming a trench capacitor and an isolating insulation film in a semiconductor device.
2. Description of the Background Art
In state-of-the-art semiconductor devices, a trench for use in forming an isolating insulation film, for example, has the following dimensions: a width of about 0.35 xcexcm and a depth of about 3.5 xcexcm. Thus, the aspect ratio of the depth to the width is about 10.
If a laser microscope using laser in an ultraviolet range is used for the trench having such a high aspect ratio, it is impossible to measure the depth of the trench since light reflected from the bottom of the trench is not detected.
The use of a scanning electron microscope (SEM) allows a user to three-dimensionally observe the trench. However, the SEM is not capable of directly measuring the dimensions, and it is hence required to employ an inaccurate technique of photographing a sample inclined at a predetermined angle, obtaining a measurement of the depth of the trench in the photograph, and correcting the measurement depending on the angle of inclination to calculate an actual trench depth.
Of course, cutting the sample at a trench portion and observing the cross-section of the trench portion through the SEM attain the measurement of the trench depth relatively accurately. This method, however, is impractical since the sample or wafer becomes unusable.
In view of the foregoing, there has typically been adopted a technique of forming test trenches to be monitored in other than a portion of the wafer wherein a semiconductor device is to be formed, e.g. in an end edge portion of the wafer, and measuring the depth of the test trenches with a configuration measuring apparatus of a probe contact type.
The test trenches formed in this technique are of several types with different widths, among which a maximum width is about 100 xcexcm because of the size of the probe for contact with the trenches. This technique comprises measuring the depth of the trenches having the respective widths by the use of the configuration measuring apparatus, obtaining a relationship between the trench widths and trench depths, and determining the depth of a trench having a width of 0.35 xcexcm by extrapolation.
As mentioned above, the background art methods indirectly measure the trench depth and are not capable of obtaining the accurate trench depth. For example, if there is a problem in a manufacturing process of a semiconductor device and a desired trench is not formed, the background art methods cause the manufacturing process to proceed without correction, to manufacture a large number of finally defective products, resulting in the decrease in product yield.
It is an object of the present invention to provide a method of directly measuring a trench depth without damages to a wafer.
According to the present invention, a method of measuring the depth of a trench formed in a main surface of a semiconductor substrate includes the following steps (a) to (d).
The step (a) mixes standard particles into a solution of cellulose acetate in acetone to prepare a molding solution, the standard particles having a uniform diameter less than the width of the trench. The step (b) drops the molding solution onto the semiconductor substrate formed with the trench, and dries the molding solution into a solid state to form a negative replica having a projection corresponding in shape to the trench and a base corresponding to the main surface of the semiconductor substrate. The step (c) peels the negative replica off the semiconductor substrate. The step (d) measures the height of the projection of the negative replica by using a vertical difference obtained by changing a relative distance between a lens of a microscope and the negative replica with respect to a focus position of the microscope.
In the method according to the present invention, the standard particles are mixed in the negative replica having the projection corresponding in shape to the trench, to make the negative replica opaque. It is, hence, easy to determine where the focus position of the microscope is. It is accordingly easy to measure the height of the projection of the negative replica by using the vertical difference with respect to the focus position of the microscope. Therefore, this method can obtain the correct depth of the trench.
Preferably, in the method, the step (d) includes the step of placing the negative replica with the projection facing toward the lens on a stage movable in small steps in a vertical direction toward the lens of the microscope, and obtaining a vertical difference between a vertical position of the stage when a tip of the projection is at the focus position and a vertical position of the stage when a surface of the base is at the focus position to determine the height of the projection.
In the method according to the present invention, the vertical difference between the vertical position of the stage when the tip of the projection is at the focus position and the vertical position of the stage when the surface of the base is at the focus position is obtained as the height of the projection. This provides a practical method of measuring the trench depth.
These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.